A 1 week old female infant with symptoms of vomiting and anorexia has a temperature of 102deg F. A bulging fontanel is noted on physical examination. The most likely agent is?

Vomiting, anorexia, high fever (above 100.4deg F), and a bulging fontanel equals neonatal meningitis until proven otherwise. Streptococcus agalactiae (group B strep) and Escherichia coli (not an answer choice) are the most common causes in neonates up to 1 month of age. The next most reasonable response would have been Listeria monocytogenes, another, though less common, cause of neonatal meningitis. Most cases of meningitis caused by Haemophilus influenzae occur in children between 6 months and 6 years of age, 90% of which result from the capsular type b strain. It has become much less prevalent since the H. influenzae type b conjugate vaccine has been routinely administered to infants. Neisseria meningitidis is the most common cause of epidemic meningitis. The two organisms most often associated with sporadic cases are Haemophilus influenzae and Streptococcus pneumoniae (the most common cause in adults over 30). Ref: Ray C.G., Ryan K.J. (2010). Chapter 25. Streptococci and Enterococci. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.